Leeway with the SunCat

The leeway angle of a sailboat depends on its overall hull design (keel, centerboard, rudder), the force of the wind on its sail, the angle of the boat to the wind, the heeling angle, the sail trimming, the shape of the sail(s),  the speed of the boat and even the wave action. Nevertheless, in most boats under normal conditions, it is customary to estimate the leeway between 5º and 15º angle. The SunCat is reputed to have a significant leeway, which is not surprising given its relative flat bottom, small keel and small centerboard and rudder (which is also a reasons for not sailing a SunCat with the sail fully trimmed  aamidships). But how much is it? A few weeks ago I set to do a series of tacks up-wind in the Ottawa River. The wind was N – NW at 8-12 kts. Waves were heading downwind and were of abt. 1-2 feet. This meant reaching up-wind in successive 20-minute tacks during which the boat averaged a SOG (speed over ground) of abt. 2.5 kts . The sail was trimmed with the end of the boom just outside the railing and I kept trying to sail with the outside telltales to the point of luffing. I estimated the angle at the bow at each tack to have been of about 100º (i.e., I’d be sailing with the bow at a 50º angle to the wind). However, the GPS COG (course over ground) track revealed something quite different: 2015-06-19 (this track was actually broadcasted in real-time to the APRS tracking radio-amateur system (see http://www.APRS.fi) using the VA3PCJ-8 callsign). The tacking angles on the picture are on average of about 144º.  However, there was an excuse, as these tacks were also being made up-current. The current in this portion of the Ottawa River is of about 0.15 knots (the water flow of the Ottawa river is of abt. 1000 m^3 /s and the section of the basin in this area is of abt. 14000 m^2, which gives a current of abt. 0.07 m/s (= 252 m/h, = 0.14 kts). This current accounts for an angle downstream of abt. 5º, which would leave an angle of about 17º ((144º – 100º) – (5º * 2)) / 2 = 17º) to be accounted as leeway on each tack. Albeit admittedly larger, this is almost within range of the usual leeway angles estimated for most cruising yachts.

From where I sit (usually in Sassy’s cockpit, tiller at hand…) having a larger leeway than other sailboats only adds to the virtues of the SunCat, since it also means longer cruising times on the water…  It also makes a case for the use of the auxiliary when the course is up-wind and time is of the essence. I’d be curious to know how these estimates compare with those that may have been made by others.

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